The present invention relates to an integrated communication system as set forth in the preamble of claim 1.
Such a modern communication system contains, besides the conventional narrow-band communication networks, i.e., the telephone network in particular, a broadband communication network which is superposed on the narrow-band networks and permits the transmission of animated pictures as are needed for video telephony and for the distribution of television programs (e.g., DE-OS No. 25 38 638). A communication system of this kind which uses optical waveguides as transmission media at least at the subscriber's level is also known under the abbreviation "BIGFON". If the existing analog or digital narrow-band telephone network is to be supplemented by a broadband network to permit broadband video communication, the following requirements should be satisfied:
1. It should be possible to convert pure telephone subscriber facilities into video-telephone subscriber facilities at any time, with no or only little advance work having to be performed in the broadband network for subscribers who wish to subscribe to the video-telephone service only at a later time or possibly not at all.
2. When the video-telephone service is introduced, the telephone subscriber should not have to change his call number; to set up a video-telephone call, he should not have to dial a video-telephone call number in addition to the telephone number.
In a telephone network, path search is commonly carried out on the basis of the call numbers, i.e., the number of a called subscriber gives information on how and in which exchanges a path has to be completed and where a subscriber with a given call number is connected to an exchange. If the telephone call numbers are to be used for a broadband switching network, and the structure of the broadband network differs from that of the telephone network, it is first necessary to determine to which point of the broadband switching network or--if there are two or more broadband exchanges--to which broadband exchange the subscriber with a given telephone call number is connected. This can be done, for example, with the aid of a table which is stored in a memory and establishes a correspondence between the telephone call numbers and the video-telephone call numbers. To be able to assign call numbers in the narrow-band network and in the broadband network in any correspondence, the video-telephone call numbers must be stored multiply, i.e., in all exchanges. This entails a certain expense in the event of a change of the call number or the subscriber line, because the call numbers stored in a multitude of exchanges have to be changed. A more advantageous solution is described in the following.